Aussie power differences shouldn't matter as the inverter runs off the 12v power supply from the machine.

Take the apron off the machine flip it over and see if the el panel is completely flush with the apron or has lost its adhesion and lifted in a few places.

I had this happen with an MM apron and found that taping any raised section down so it sits totally flat against the underside of the apron cut down on the buzzing noise.

I'm guessing it had something to do with needing proper grounding the whole way around the el panel/metal apron.

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Coin door shut? Connector plug around the right way?

I'm away for Easter/Anzac but if you are still stuck I'll check an apron out from one of mine when I get back.